Using update-alternatives to change JVM

I install openjdk-7-jdk.
Side effect of installing Grails using repository
After I install grails using the command sudo apt-get install grails The side effect of the installation is that change the default jvm, openjdk-7-j-jdk in my case, to the openjdk-6-jdk. If you want reenable the jvm, you can use the command sudo update-alternatives --config java